Properties and Preparation Reactions of Common Synthetic Polymers

To determine the correct statement(s) regarding polymers, let us analyze each option individually based on NCERT polymer chemistry concepts:

1. Option (A):

   • Polymerization of chloroprene ($2\text{-chloro-1,3-butadiene}$) yields Neoprene (polychloroprene), which is a synthetic rubber.
   • Natural rubber, on the other hand, is a natural polymer of isoprene ($2\text{-methyl-1,3-butadiene}$), specifically cis-1,4-polyisoprene.
   • Thus, statement (A) is incorrect.

2. Option (B):

   • Teflon (Polytetrafluoroethene or PTFE) is prepared by heating tetrafluoroethene ($\text{CF}_2=\text{CF}_2$) under high pressure in the presence of a persulphate catalyst (e.g., ammonium persulphate): $n\,\text{CF}_2=\text{CF}_2 \xrightarrow[\text{High pressure}]{\text{Persulphate catalyst}} \text{--[CF}_2\text{--CF}_2\text{]--}_n$
   • Thus, statement (B) is correct.

3. Option (C):

   • Thermoplastic polymers are linear or slightly branched long-chain polymers capable of repeatedly softening on heating and hardening on cooling due to moderate intermolecular forces.
   • Polyvinyl chloride (PVC) is a well-known example of a thermoplastic polymer.
   • Thus, statement (C) is correct.

4. Option (D):

   • Polymerization of ethene at $350\text{--}570\text{ K}$ and $1000\text{--}2000\text{ atm}$ pressure in the presence of a peroxide initiator proceeds via a free-radical mechanism to yield Low-Density Polythene (LDP).
   • High-Density Polythene (HDP) is formed at a lower temperature ($333\text{--}343\text{ K}$) and lower pressure ($6\text{--}7\text{ atm}$) using a Ziegler-Natta catalyst ($\text{Et}_3\text{Al} + \text{TiCl}_4$).
   • Thus, statement (D) is incorrect.

Conclusion: The correct statements are (B) and (C).